Since I saw the original trilogy first, I knew who he really was the moment I saw his hooded hologram in Ep. I.
Since Bane instituted the Rule of Two, it was tradition that the apprentice slained the master after he/she became more powerful than the master. Therefore, the new generation was always stronger then the previous - except Vader, he was weaker. As I read many times, Sidious was the most powerful Sith Lord of all time, "the Dark Side itself". A Dark nexus.